Nest-making orangutans build for comfort and strength

Many great apes build nests to sleep in at night, and these structures are an …

Great apes are our closest relatives, and they are constantly surprising us with their human-like abilities and tendencies; here at Ars, we've seen studies showing that they plan ahead, they engage in wars over territory, and they respond to being tickled, just to name a few. Now, a new study in PNAS adds to the growing list of ways in which great apes are similar to us: they value a good night’s sleep, and they have the technological know-how to build pretty impressive beds from some very basic materials.

Many species of great ape, including chimpanzees, gorillas, bonobos, and orangutans, build nests in the forest canopy. Generally, these nests are slept in for just one night, then abandoned. The nests are thought to benefit the apes in several ways: sleeping above the ground reduces disturbances, protects them from predators, leaves them less vulnerable to mosquitoes and other pests, and generally increases the quality of their sleep. However, we know very little about how these nests are actually constructed. To figure out how much skill goes into nest-building, a group of scientists studied nests made by Sumatran orangutans in Indonesia.

The orangutans' nests are generally built between 11 and 20 meters off the ground, and are built entirely from branches, sticks, and leaves. Since female orangutans average more than 38 kilograms, the nests have to be pretty sturdy. So how do the apes build nests that are not only comfortable, but robust as well?

It turns out that orangutans follow a similar pattern of construction for all their nests. After choosing a horizontal branch or branches to build the nest on, the apes bend particular branches inwards and weave them together. Smaller, broken branches are then layered on top of this basic structure and woven into a sort of "mattress." Then, leafy branches are added as a lining. Finally, depending on the individual and the nest, roofs or leafy pillows are sometimes added.

While the steps seem simple, it's the way the branches are chosen and manipulated that makes an ape's nest an impressive technological feat. Some of the branches that the orangutans use are living, and are difficult to break cleanly (if you've ever tried to bend and break a green branch, you probably appreciate the difficulty of this task). To skirt this difficulty, and to take advantage of these particular branches' strength, the orangutans use them as the main structural elements of the nests, bending them in toward the middle of the nests and weaving them together (you can see a short clip of this process here).

Sticks that are easier to break cleanly are used in the lining of the nest, and in other parts of the structure where strength is less vital. The researchers found that the structural support branches are four times more rigid and four times stronger than the branches used for the mattress. From these results, it seems that orangutans are choosing branches with certain structural qualities for different parts of the nests.

The orangutans' construction skills enable the center of the nest to be more flexible, or "compliant," than its edges. In fact, the nests are amazingly stretchy: the center of a nest is able to flex more than one meter downwards to accommodate a heavy ape. Around the rim, where strength is more important than flexibility, the nest has much less give.

Previous studies have shown that there is a genetic component to nest-making; that is, without any guidance from other apes, an individual will build nests. However, young apes that are exposed to other nest-building adults are much more efficient builders and tend to create stronger nests. This information cements the idea that apes are making conscious decisions about the construction of their nests.

Some researchers suggest that the knowledge and technological innovation involved in great ape nests may be a precursor to tool use in these species, since both skills require higher order cognition and representational thinking. It may be no coincidence that birds—the consummate nest-builders—are also among the best tool users in the animal kingdom.

18 Reader Comments

Sounds like some of these apes should go work for Simmons Mattress. Maybe they can teach their engineers a thing of two. My bed was comfy at first then became horrible over a couple of months. It for sure needs more comfort and strength.

So this could mean that humans ancestors were terrible nest builders. Forcing us down to walk the plains.

More likely it means that before we could sleep on the ground -- in plains that had lions, hyenas and leopards stalking around at night -- we had to invent fire. ("Invent fire" is my shorthand for "learn how to consistently use fire as a tool".)

But that would have serious implications for the time-line of fire invention. Pre-human ancestors are assumed not to have regularly used fire as a tool. But in spite of having arms adapted for climbing, pre-human upright walkers lost the ability to hold on to tree limbs with their feet -- thought to be a necessary skill for building aerial nests.

Here are the proposed hypotheses for how pre-human (but upright walking) ancestors slept:

(i) They slept in trees without using nests. Possible, but potentially dangerous and uncomfortable.

(ii) They were able to build aerial nests without having feet that could grip tree limbs. Possible, especially if social groups helped each other build nests, but seems very inefficient. Try to picture how you would build a nest of tree branches without being able to hold on with your feet, even if someone else helped. It's not just enough to sit on the branch, since apes tend to use their feet not just to hold on, but as part of the bending/weaving process as well.

(iii) They slept on the ground, escaping to trees if danger approached. Possible, but hazardous.

(iv) They already invented fire and used camp fires to keep predators away. Intriguing idea that would push back the assumed invention date of fire by a million years or more.

So this could mean that humans ancestors were terrible nest builders. Forcing us down to walk the plains.

More likely it means that before we could sleep on the ground -- in plains that had lions, hyenas and leopards stalking around at night -- we had to invent fire. ("Invent fire" is my shorthand for "learn how to consistently use fire as a tool".)

But that would have serious implications for the time-line of fire invention. Pre-human ancestors are assumed not to have regularly used fire as a tool. But in spite of having arms adapted for climbing, pre-human upright walkers lost the ability to hold on to tree limbs with their feet -- thought to be a necessary skill for building aerial nests.

Here are the proposed hypotheses for how pre-human (but upright walking) ancestors slept:

(i) They slept in trees without using nests. Possible, but potentially dangerous and uncomfortable.

(ii) They were able to build aerial nests without having feet that could grip tree limbs. Possible, especially if social groups helped each other build nests, but seems very inefficient. Try to picture how you would build a nest of tree branches without being able to hold on with your feet, even if someone else helped. It's not just enough to sit on the branch, since apes tend to use their feet not just to hold on, but as part of the bending/weaving process as well.

(iii) They slept on the ground, escaping to trees if danger approached. Possible, but hazardous.

(iv) They already invented fire and used camp fires to keep predators away. Intriguing idea that would push back the assumed invention date of fire by a million years or more.

you left out they lived in caves which having only one entrance could be easily defended. Also left out was making a kind of nest out of prickly branches that predators in the night would prefer to avoid something still done by bushwalkers today. neither of which would require the invention of fire.

you left out they lived in caves which having only one entrance could be easily defended. Also left out was making a kind of nest out of prickly branches that predators in the night would prefer to avoid something still done by bushwalkers today. neither of which would require the invention of fire.

While we find a lot of archaeological evidence in caves, that has more to do with the preservative nature of caves than their importance to human evolution. There just aren't enough caves for humans to have evolved in a way that would be dependent on them. Especially true in the savannahs where it is believed upright walking first evolved.

As far as prickly branches... when you mention bushwalkers, are you talking about Australian aboriginals? or just modern wilderness hikers? There are problems with either example. There are no large predators on Australia, so anything the aboriginals do to keep dingoes and monitor lizards away from camp would not necessarily apply in the African plains. Modern wilderness hikers only need to slow down predators long enough to employ modern weapons or scare tactics (shining flashlights, noisemakers, etc.) against predators. Remember, also, that when discussing early human ancestors that walked upright, we're talking about creatures the size of a chimpanzee who probably had not invented throwing-weapons -- not 5+ foot tall homo sapiens with stone-tipped spears.

you left out they lived in caves which having only one entrance could be easily defended. Also left out was making a kind of nest out of prickly branches that predators in the night would prefer to avoid something still done by bushwalkers today. neither of which would require the invention of fire.

While we find a lot of archaeological evidence in caves, that has more to do with the preservative nature of caves than their importance to human evolution. There just aren't enough caves for humans to have evolved in a way that would be dependent on them. Especially true in the savannahs where it is believed upright walking first evolved.

As far as prickly branches... when you mention bushwalkers, are you talking about Australian aboriginals? or just modern wilderness hikers? There are problems with either example. There are no large predators on Australia, so anything the aboriginals do to keep dingoes and monitor lizards away from camp would not necessarily apply in the African plains. Modern wilderness hikers only need to slow down predators long enough to employ modern weapons or scare tactics (shining flashlights, noisemakers, etc.) against predators. Remember, also, that when discussing early human ancestors that walked upright, we're talking about creatures the size of a chimpanzee who probably had not invented throwing-weapons -- not 5+ foot tall homo sapiens with stone-tipped spears.

I am talking about africans living in the exact same type of savannah you mentioned facing the same predators you mention. I saw a nature program about african hunters and how when they have to sleep in the open they gather prickly bushes and form them into a small enclosed circle to protect them from night hunters such as lions hyennas that prefer not to get shredded by prickly bushes to get a meal. This method is used as an alternative to fire which in a hot country could be less comfortable to sleep beside.

you left out they lived in caves which having only one entrance could be easily defended. Also left out was making a kind of nest out of prickly branches that predators in the night would prefer to avoid something still done by bushwalkers today. neither of which would require the invention of fire.

While we find a lot of archaeological evidence in caves, that has more to do with the preservative nature of caves than their importance to human evolution. There just aren't enough caves for humans to have evolved in a way that would be dependent on them. Especially true in the savannahs where it is believed upright walking first evolved.

As far as prickly branches... when you mention bushwalkers, are you talking about Australian aboriginals? or just modern wilderness hikers? There are problems with either example. There are no large predators on Australia, so anything the aboriginals do to keep dingoes and monitor lizards away from camp would not necessarily apply in the African plains. Modern wilderness hikers only need to slow down predators long enough to employ modern weapons or scare tactics (shining flashlights, noisemakers, etc.) against predators. Remember, also, that when discussing early human ancestors that walked upright, we're talking about creatures the size of a chimpanzee who probably had not invented throwing-weapons -- not 5+ foot tall homo sapiens with stone-tipped spears.

The Masai tribes in Africa surround their houses/cattle with fences made out of prickly bushes, the purpose of using that is to keep large predators such as lions and hyenas from making a meal out of them or their cattle. It's most likely that our early ancestors learned that trick too.

Kate Shaw Yoshida / Kate is a science writer for Ars Technica. She recently earned a dual Ph.D. in Zoology and Ecology, Evolutionary Biology and Behavior from Michigan State University, studying the social behavior of wild spotted hyenas.